This invention concerns lustrous pigments and processes for producing the same. More specifically, it relates to lustrous pigments which are useful in various fields such as for synthetic resin, cosmetics, ink, synthetic leather, wallpaper and the like, as well as to processes for producing such pigments.
Natural pearl essence, basic lead carbonate, bismuth oxychloride, titanium dioxide-coated mica and the like are examples of lustrous pigments. However, commercial demand is limited for natural pearl essence because of the cost, depending on the yields and of the unstable quality which may vary depending on the clarity of colors. The demand for lead carbonate and bismuth oxychloride is limited because of their toxic nature. Thus, the titanium dioxide-coated mica pigments are used predominantly at present, because they are excellent in view of their non-toxicity and stabilized physical properties.
As a lustrous pigment, however, the titanium dioxide-coated mica still has some disadvantages. For instance, although the grain size is considered to be one of the major factors that influence the quality of titanium dioxide-coated mica pigment, it is difficult to make the grain size uniform for the flaky thin mica pigment obtained by pulverizing and classifying natural mica as the main raw material, and the technique therefor has not yet been established. Also, the method of use and the application field for the pigment are limited at present since the products lack physical strength.
It has been reported that mica-like or plate-like iron oxide (micaceous iron oxide, hereinafter referred to as MIO) is produced by applying hydrothermal treatment (or treatment under pressure in an aqueous environment, such as in an autoclave) to iron oxyhydroxide or a colloidal precipitate of ferric hydroxide in aqueous alkaline solution. The resultant synthesized MIO exhibits rather intense metal luster (Japanese Patent Publication No. 12435/1968). Although MIO is highly regarded as an excellent anti-corrosion paint for iron and steel structures, MIO lacks a decorative effect, due to its blackish-purple color, as contrasted to the clear color desired for the anti-corrosion paints that have been used as coatings on iron and steel structures. Therefore, MIO is not used as over-coating, but, in most cases, as an undercoating, utilizing the anti-corrosive effect. Where MIO paints are used alone, they are applied only to those limited locations such as the underside of the bridges where the colors of the paints are considered insignificant. As one of the methods for giving various colors to the MIO paints, Japanese Patent Publication No. 7176/1976 discloses a method of coating the MIO paints with transparent metal oxides having high refractive index, such as titanium dioxide and zirconium dioxide, which exhibit interference colors such as purple, blue and green. This method cannot, however, provide as clear a color but can produce only those pigments inferior in decorative effect, as compared with titanium dioxide-coated mica pigment.